Protection of metallic objects by galvanic action



Patented Mar. 1 1949 PROTECTION oF vis'rALLIo OBJECTSBY VGALVANIOACJII'ON {can Frasch, also known as Herbert Manfred Freud, Clichy, France No Drawing. Application January 17, 1940, Serial No. 314,349. "In France January 31," '1 9 3 9' In the copending U. S'. patents application serial Number 297,191 entitled Improvements in process for the'protection'of magnesium and its alloys, filed on September 29, 1939, which has became abandoned, thefpresent inventor has de- 9 Claims. (01. 204-56) scribed a process for producing a coating resistant to corrosion on magnesium-base objects, which consisted in employing an acid bath character- "ised by the presence of an ion containing the "elernen't manganese. In the aforesaid patent application it i's stated thatthe invention is p-reierably carried out by means of alternating current, but that it is also possible to procure the protective coating in question by purely chemical means. In this latter case the chemical action is defined as being obtained by immersion in the bath of the n agesium-base objectto be protected without the said object forming any part of an electric circuit through which current passes.

The inventor on continuing his researches has nowdiscovered that the presence in the bath of an ion containingthe element manganese is capable :lof giving rise to galvanic phenomena resulting in the deposition not only on magnesium and its alloys but also on other metals, of a protective coating more resistant to corrosion than any which have heretoforebeen obtained. In other words if one connects, in the absence of any'extraneous source of electric current, the object to be treated to a second electrode in the bath by means of an electric conductor outside the bath,

an electric current is set up which ceases 'if the electric connection is broken. The system functions as a primary cell, and one obtains in this way, the aforementioned excellent coating so highly resistant to corrosion.

The new invention consequently consists in immersing the metallic object to be protected, after suitable degreasing and pickling, in a bath containing ions wholly or partially constituted of for example be composed of a magnesium-base metal, is then electrically connected, as explained above, to a second electrode composed of a more noble metal or element, such for instance as lead,

iron or especially carbon, without the adjunction of any extraneous source of current. A galvanic current is at on-ce'set up, accompanied by a deposit of an oxide of manganese on the magnesium base electrode." Ashas been stated, this coating is particularly resistant to corrosion.

"ifrne manganese maybe present inlthe bath either in theformnfmanganeseions formed by the dissociation of a manganous salt, or in the form of manganate or permanganate ions. It is 1 manganese. The said metallic object, which may 2 particularly advantageous that the ion containing the elementmanganes e should be associated with"chr"omate or bichr'omate ions, agrar anstance in MnCreOi. 5 Theinventorhas further discovered that all soluble manganates and permang'anates-are capable-of givingriseto the ab'ove described protective coating, whereas-not all solubl mahg astance'nitrates, chlorides andisulpha-ts are incapable of forming adequate protective coatings on magnesium-basernetals, sin'cethey attack the metal. By'such attack'on the metal, some-of? the magnesium (irom the surface of thefm'agnesium or magnesiumbase alloy) would become" dissolved intne b'ath,andjsoine of it w'ouldfbe found the coating produced galvanically in the process, whichlatterwould give acoating'oi inier'ior qual- In the case of voluminous metal objects which it is diiilicult to treat me bath; the inventbr'has which the metal or metals to he "prctectedenter. The Coating is fdlm d Of all bldde OI OXi dS Qf manganese, whichmay also beassdciat'e'd with an oxide of metals whdseichs are present together with manganese i'nth bath.

' Thep'i-oteetioifaficrded by the coating so produced can be still further increasedby tr'e'atfnent in a bath of hot paraflin at temperatures o'fabout to 200 0., also by treatment with other hot heavy hydrocarbons. 'Tfhe' inventor has "discovered tohis surprise that if ametallic object, for ex'amplean object composed ofa magnesium alloy, is treated as "set f orth ab'c'iveand then dipped in a bath of'hot paraffin, a violent chemical reaction takes place, as is evidenced by "a'copious evolution of gas" At' the sameness structure of the protective coating is changedfandl'the colour deepens to black. It appears "thatfthe oxide of oirides'forrningthe protective 'filin will temperatures stated, and hydrated compounds will undergo dehydration.

The following examples, which are given by way of example only, and which must not therefore be construed in any limiting spirit, will make clear exactly and in what way the above invention is to be carried out.

Example 1.A piece of sheet composed of a magnesium alloy containing 1.5% manganese, is cleaned mechanically and immersed for one hour in a 1% solution of KMnOr, and at the same time connected electrically to a second electrode of carbon. The temperature of the solution is maintained at 60 C. One obtains a light brown film of an oxide of manganese.

Example 2.An object composed of a magnesium-alloy containing 3% Al and 1% Zn which object has been forged, is pickled in a solution of chromic acid and immersed at room temperature for 4 hours in a solution of MnCrzOv, and at the same time connected electrically to a second electrode of carbon. One obtains a dark brown coating, which takes on a black aspect after treatment in a bath of hot paraflin at 180 C. The film contains an oxide of manganese together with an oxide of chromium.

Example 3.-A cast object composed of magnesium alloy containing 10% Al, is first pickled in nitric acid and then immersed for 2 hours at room temperature in a solution made by dissolving 50 grams of C1303, 10 grammes of KMnO4 and 50 grammes of MIiCOs in 1 litre of water, the said object being at the same time electrically connected to a second electrode of iron. The film obtained is grey.

Example 4.-An object composed of semi-mild steel, after previous picking in H2SO1, is immersed for half an hour at 40 C. in a solution obtained by dissolving 100 grammes of MnCOz in a litre of water containing 70 grammes of CrOs, the said object being at the same time connected electrically with a second electrode of lead. One obtains a grey film which turns black after treatment in hot paraflin.

The coatings so formed are very adherent, and can be obtained very thick if one continues the galvanic treatment for a suificient length of time. The coating is moreover of substantially uniform thickness and character irrespective of the shape of the object treated. This is true even for the interior of angles or tubes in contradistinction to the well known difiiculty of obtaining uniform deposits on such parts in the case of the anodic processes of protection habitually employed. It is highly probable that this result is due to the nature of the current giving rise to the deposit, this current being galvanic and not electrolytic in origin; in other words the process itself produces the electric current necessary to form the deposit, instead of consuming an electric current extraneously produced.

The above invention also covers as new industrial products all metallic objects, and in particular objects made of magnesium-base metals, on which coatings composed principally of an oxide of manganese have been deposited, either as such or when subsequently treated in a bath of hot parafiin or other hydrocarbon, the galvanic baths serving to put the said invention into effect and the protective coating itself so obtained.

I claim:

1. A process for protecting metallic objects composed of a substance selected from the group consisting of magnesium and alloys containing magnesium as their major constituents, by making the object to be protected the anode, in a galvanic battery having a cathode more noble than magnesium and having an electrolyte consisting essentially of an aqueous solution of at least one compound selected from the group consisting of manganates, permanganates and mananese bichromate, electrically connecting said anode and said cathode outside said bath, and allowing a galvanic current to flow, in said solution, from said anode to said cathode until a protective film of substantial thickness is formed on said anode, removing said anode from said solution and washing same, all without supplying any electric current from the outside of said galvanic cell.

2. Process according to claim 1 in which the electrolyte is an aqueous solution of a manganate.

3. Process according to claim 1 in which the electrolyte is an aqueous solution of a permanganate.

4. Process according to claim 1 in which the electrolyte consists essentially of an aqueous solution of manganese bichromate.

5. An article formed of a substance selected from the group consisting of magnesium and magnesium alloys containing magnesium as the major component, and having a coating consisting essentially of an oxide of manganese and an oxide of chromium, which coating is brown to black in color and which coating is devoid of magnesium compounds, such coating being very adherent to the magnesium and very resilient to corrosion, and of substantially uniform thickness.

6. A process as in claim 1, in which the electrolyte is a solution of MnCr2O7 of about 10% concentration.

7. A process as in claim 1, in which the electrolyte is a solution of permanganate of about 1% concentration.

8. A process as in claim 1, in which the electrolyte is a solution of manganese bichromate and a permanganate.

9. A process as in claim 1, in which the electrolyte is an aqueous solution made by adding 50 gr. MnCOa, 50 gr. ClOs and 10 gr. KMnO4 to each liter of water.

JEAN FRASCH. Also Known as Herbert Manfred Freud.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 737,882 Strecker Sept. 7, 1903 1,552,591 Batenburg Sept. 8, 1925 1,952,850 Kochler Mar. 27, 1934 1,974,435 Schulz et al Sept. 25, 1934 2,052,962 Booe Sept. 1, 1936 2,138,023 Buzzard Nov. 29, 1938 2,203,670 Buzzard June 11, 1940 2,206,028 Buzzard July 2, 1940 2,224,528 Sutton et a1 Dec. 10, 1940 FOREIGN PATENTS Number Country Date 493,935 Great Britain Oct. 17, 1938 702,266 France Jan. 20, 1931 471,053 Germany Feb. 6, 1929 Certificate of Correction Patent No. 2,463,483. March 1, 1949. JEAN FRASGH, also known as HERBERT MANFRED FREUD It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 4, line 1, claim 1, for the word constituents read constituent; line 33, claim 5, for resilient read resistant;

and that the said Letters Patent should be read with these corrections therein that the 'same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 17th day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant Gammz'ssz'oner of Patents. 

